Atrial fibrillation (AF) is the most common adult cardiac tachyarrhythmia affecting more than 2,200,000 people in the US. AF also accounts for 15% of all strokes. Ectopic electrical impulses originating in the pulmonary veins initiate AF in many patients. Surgical or catheter-based procedures that isolate pulmonary veins from the remaining atrial tissue may be offered to those patients who fail from medical therapy or suffer intolerable side effects. Surgery is invasive, risks morbidity and mortality and requires long recovery. In response, novel minimally invasive, catheter-based radiofrequency (RF) ablation approaches are being developed. However, catheter ablation guided by X-ray fluoroscopy (XRF) with adjunctive electroanatomic/ mechanical mapping (EMM) is challenging and time-consuming. Limitations with XRF include poor soft-tissue contrast and poor depth perception due to 2D projection imaging. Catheters are "blindly" steered in the heart chambers without visualization of endocardial surfaces or wall thickness, risking inappropriate ablation, life-threatening perforation or valve trauma. EMM requires expert skill and can take hours to obtain a low-resolution static roadmap. Cardiac, respiratory or random patient movements are not resolved with this or other roadmap techniques. For these reasons, catheter-based pulmonary vein ablation using conventional imaging is not widely attempted. Alternatively, magnetic resonance imaging (MRI) offers superior soft-tissue contrast with imaging in any arbitrary plane. Recent advances in novel therapeutic devices and rapid imaging techniques have made catheter-based interventions possible. Specific advantages for real-time MRI-guided cardiac ablation over conventional image guidance include: 1) real-time, high tissue contrast, high resolution visualization of heart chambers and pulmonary veins in any arbitrary spatial perspective 2) real-time catheter navigation and positioning overcoming respiratory, cardiac and random patient movement (like non-roadmap imaging), 3) spatial and temporal tracking and visual assessment of ablation zones, 4) functional imaging to evaluate atrial and cardiac physiology, and flow dynamics during therapy, 5) elimination of radiation exposure to both patients and operators. We propose to design, develop, and validate a novel catheter-based pulmonary vein ablation approach using catheters modified with "multi-mode" intravascular MR probes with tracking, imaging, wireless marker, and RF ablation features integrated into a 3D real-time MRI guidance system. Successful completion of this project is expected to improve safety, simplicity, and efficacy catheter ablation procedure to treat AF.